1. Field of the Invention
The present invention concerns a method for controlling a magnetic resonance system for implementation of a magnetic resonance measurement in at least one specific volume region within an examination subject, wherein the magnetic resonance system has a radio-frequency antenna configuration with a number of individually controllable transmission channels for generation of radio-frequency fields in an examination volume surrounding the examination subject. The invention also concerns a method for generation of a collection of examination situation classes. Moreover, the invention concerns an antenna control device for implementation of such a control method as well as a magnetic resonance system with a corresponding antenna control device.
2. Description of the Prior Art
Magnetic resonance tomography is a technique for acquisition of images of the inside of the body of a living examination subject that has achieved widespread use. In order to acquire an image with this modality, the body or a body part of the patient or test subject to be examined must initially be exposed to an optimally homogeneous static basic magnetic field, which is generated by a basic field magnet of the magnetic resonance system. Rapidly switched gradient fields that are generated by gradient coils are superimposed on this basic magnetic field for spatial coding during the acquisition of the magnetic resonance images. Moreover, radio-frequency pulses of a defined field strength (known as the “B1 field”) are radiated into the examination subject with radio-frequency antennas. The nuclear spins of the atoms in the examination subject are excited by these radio-frequency pulses, such that they are deflected from the equilibrium state parallel to the basic magnetic field by what is known as an “excitation flip angle”. The nuclear spins then precess around the direction of the basic magnetic field. The magnetic resonance signals thereby generated are acquired by radio-frequency acquisition antennas. The magnetic resonance images of the examination subject are generated on the basis of the acquired magnetic resonance signals.
To emit the required radio-frequency pulses in the patient positioning region, the tomography apparatus typically has an antenna structure permanently installed in the scanner housing. This radio-frequency antenna is also designated as a “body coil”. It is formed (for example in the frequently used “birdcage structure”) of a number of conductor rods arranged around the patient space and running parallel to the primary field direction, the conductor rods being connected with one another by ferrules at the facing ends of the coil. There also can be other antenna structures permanently installed in the housing (such as saddle coils, for example). Classical magnetic resonance systems essentially have only one transmission channel for emission of the B1 field, meaning that there exists only one transmission line that leads from the radio-frequency amplifier to the antenna structure. Insofar as the antenna (such as, for example, birdcage antenna) is fashioned such that a circular polarized field can be emitted, the radio-frequency signal to be emitted (which arrives from the radio-frequency amplifier) is split by a hybrid module into two signals that are shifted opposite to one another by 90° in terms of their phase. The two signals are then fed via two transmission lines into the antenna structure at precisely defined connection points. The distribution of the B1 field is permanently “frozen” by the division to the two transmission channels with the phases of 0° and 90° and cannot be adapted to the current conditions of the present measurement. Moreover, local coils also can be used that are arranged directly on the body of the patient. These coils have normally been used only as acquisition coils.
In particular in newer novel magnetic resonance systems with basic magnetic field strengths greater than three Tesla, considerable eddy currents are frequently induced in the patient upon radiation of the radio-frequency pulses. The actual homogeneously radiated B1 field is consequently more or less strongly distorted. The influence of the patient body on the B1 field is thereby dependent on, among other things, the stature (body type) of the patient and the proportions of the individual tissue types. For example, a very corpulent patient causes a circularly polarized magnetic field to be strongly distorted into an elliptical field. By contrast, this distortion is not so severe in thinner patients. In individual cases this can lead to the situation that a reliable magnetic resonance measurement is problematic in specific body regions of the patient and delivers unusable results.
In order to be able to influence the structure of the radiated magnetic field in a suitable manner with optimal detail in all regions of the examination volume, and in particular in order to achieve an optimally good homogeneity of the B1 field in an examination volume by compensation of the possible distortions, local field corrections have previously been implemented by the use of (for example) dielectric cushions.
Individual adjustments of the amplitude values and the phase values of the radio-frequency pulse emitted by each transmission channel are presently under discussion as a further promising approach for homogenization of the B1 field. The spatial distribution of the B1 field can thereby be influenced with the goal of generating an optimally homogeneous radio-frequency field in the examination subject, or in the examination volume with consideration of the field distortions to be expected. One design is the use of a number of separately controllable antenna elements. An example of this technique is explained in DE 101 24 465 A 1, which describes an antenna with a number of separately controllable antenna elements. For this purpose, each transmission channel has a separate antenna element. Moreover, various feed lines connected to the overall antenna structure (for example the aforementioned two feed lines to a birdcage structure for emission of a circularly polarized field) can also be supplied via individually controllable transmission channels.
An unsolved problem exists in determining the antenna control parameter values for the individual transmission channels in a fast and simple manner so that the desired B1 distribution is achieved in the patient or at least in the area of interest (region of interest, ROI) for the present acquisition. An adjustment known as a static B1 adjustment is presently implemented in such apparatuses to determine the parameters. Such adjustments, however, are extraordinarily time-consuming and therefore are not very suitable in practice in many cases.